1. Field of the Invention
The invention is directed to a heat exchange apparatus and a method for cooling various gaseous substances, and particularly to the indirect evaporative cooling of air.
Presently, refrigeration machines are predominantly used for air cooling. Although generally effective in cooling air, such machines consume considerable amounts of energy, have a complicated design, and can be unreliable.
As an alternative to refrigeration machines, methods for evaporative cooling of air have been proposed. Evaporative cooling takes advantage of the natural non-equilibrium of ambient air. The psychrometric difference of air temperatures is used as the source for air cooling, i.e., the difference between the dry bulb temperature and the wet bulb temperature.
For evaporative air cooling, air unsaturated with water vapor is contacted with water. This causes some of the water to evaporate into the air where heat is transferred from the air to the evaporating water. This heat is then returned to the air as latent heat of vaporization. This results in the air temperature being reduced. Theoretically, the temperature can be reduced to the wet bulb temperature of the ambient air.
Although evaporative cooling can be effective in some circumstances, the method of evaporative air cooling has a number of serious drawbacks. One important drawback is the narrow limit of the cooling operation (the wet bulb temperature of the ambient air). Moreover, in evaporative air cooling the ambient air becomes humidified when contacting the water. However, the air's heat content does not vary. In other words, the process of evaporative cooling yields no actual refrigeration, as evident heat is merely converted into latent heat, i.e. the enthalpy of the air remains constant. Hence, air thus humidified and cooled has little value for utilization in air conditioning units.
The process of evaporative air cooling may be feasible in limited climatic zones where the exceedingly low moisture content of ambient air, and its humidification in the process of evaporative cooling, would not cause substantial humidification of air. However, even some humidification may adversely affect the cooling properties of such air when used in air conditioning systems.
A method for indirect evaporative cooling has been proposed as described in USSR Author's Certificate Number 615319, the disclosure of which is hereby incorporated by reference. According to the proposed evaporative cooling method, air is cooled when contacting a heat exchange surface having an opposite side that is moistened with water. Directed over the moistened side of the heat exchange surface is a stream of ambient air. While passing over the moistened side, the ambient air becomes humidified and cooled to approach the wet bulb temperature by the process of evaporative cooling. This process allows the heat exchange surface from which moisture evaporates into the ambient air to be cooled and leads to the cooling of air contacting the opposite side of the heat exchange surface, i.e. the air on the dry side of the heat exchange surface is indirectly cooled.
The method of indirect evaporative cooling also utilizes the natural imbalance of atmospheric air, i.e., the psychrometric temperature difference. As previously described, a limitation of evaporative cooling is that the extent of cooling is limited to the wet bulb temperature of ambient air. In practice, this limitation is even greater with indirect cooling because of the thermal resistance of the heat exchange surface. In addition, the effectiveness of the air cooling is limited due to the low intensity of heat transfer processes between heat exchanging flows of air.
Another method for indirect evaporative cooling has been proposed in U.S. Pat. No. 4,002,040. According to this method, a stream of ambient air is directed through a first group of channels, the walls of which are wetted. As the air passes through these channels, water from the walls is evaporated into the air, causing the air to become humidified and causing the temperature of the channel walls to decrease. Another group of channels are situated against the walls of the first group of channels so that the two sets of channels are in thermal communication. Through the second group of channels, a second stream of air is directed. Since the walls of the second channel are in thermal communication with the walls of the first channels, the second stream of air directed through the second channels is indirectly cooled due to the evaporation occurring in the first channels. As previously described, a limitation to this type of indirect evaporative cooling is the wet bulb temperature of ambient air. Moreover, the degree of cooling is further limited on account of the thermal resistance of the channel walls.
2. Description of the Background Art.
As previously described, USSR Author's Certificate Number 615319 and U.S. Pat. No. 4,002,040 describe methods for indirect evaporative cooling.